This invention relates to earth boring rotary cone rock bits used in oil field applications. These bits have a body with two or more journal segment arms with rotary cone cutters mounted thereon. The cutters are mounted on bearing pin shafts which extend downwardly and inwardly from the journal segment arms. These bits are conventionally attached to hollow drill pipes and suspended downwardly from a drilling rig at the surface. Rotational energy and weight applied to the bit by the drill pipe force the rotary cutters into earth formations. Borehole is formed as the punching and scraping action of the rotary cutters remove chips of formation. These chips are carried away by fluid forced down through the drill pipe and bit. The fluid carries chips and cuttings with it as it flows up and out of the borehole.
The rate at which borehole is formed is largely a result of the design of the rotary cutters. There are two main categories of rotary cutters; milled tooth cutters and tungsten carbide insert (TCI) cutters. The teeth on milled tooth cutters are integral parts of the cone and are formed by a milling operation, hence the name. The teeth on TCI cutters are made of tungsten carbide and are press fit (inserted) into undersize apertures on the cone. The teeth on the cutters functionally break up the formation to form new borehole by punching into it vertically and scraping horizontally. The amount of punching action is governed primarily by the weight on the bit. The horizontal scraping motion is a resultant of the position and shape of the cone cutter.
Medium and soft formation bits usually drill through varied formations in a single well. Recording devices which show instantaneous rates of penetration will often show rates as high as four feet per minute and rates as slow as one foot in ten minutes on the same bit run. As a rule, the formations tend to become harder as depth increases but there are large variations in hardness at all depths.
Bits having long inserts are most efficient for fast drilling in soft formations. In the very soft formations the penetration rate of a bit is limited by the length of its inserts. When the full length of the insert penetrates into the formation the steel body of the cone forces against the formation and limits further penetration. Long inserts are relatively weak though, and are subject to breakage in the slower drilling hard formations. Short blunt inserts are better suited for the harder formations because they are less subject to breakage, but they limit a bit's penetration rate in soft formations. Numerous attempts have been made to reduce the insert breakage without compromising the penetration rate of the bit. Examples are shown in U.S. Pat. No. 4,108,260 to Bozarth, U.S. Pat. Nos. 4,334,586 and 3,495,668 to Schumacher, and U.S. Pat. No. 3,696,876 to Ott. All of these attempted to prevent or reduce the breakage by making the points of the inserts blunter.
On most modern tri-cone rotary rock bits the cones are positioned such that the axis or centerline of the cones do not intersect the centerline of the borehole and rock bit. They are offset with the cone centerline leading the bit centerline. (Leading and trailing are common rock bit terms used to describe positions. The side of an object that is facing the direction of rotation is referred to as the leading side. The side of an object that is facing opposite the direction of rotation is referred to as the trailing side.) U.S. Pat. No. 3,495,668 to Schumacher discloses a bit with offset or skewed roller cutters. This offset causes any point on a cone to be farther from the bit centerline before that point touches the bottom of the borehole than after. This offset position of the cone cutter causes any insert engaging formation to scrape inboard or toward the bit centerline as the bit rotates.
The arcuate shape of the cone causes circumferential drag of the inserts. Each row of inserts would have a different rotational rate based on the diameter of each row and the distance of each row from the center of the bit if each row were free to rotate independently. Because the rows are locked together the inserts of some rows will scrape toward the leading side and the inserts of the other rows will scrape toward the trailing side.
The theoretical horizontal scraping motion of the inserts inboard and circumferentially can be calculated. However, the actual rotation rate of a cone is a resultant of the forces acting on each insert embedded in formation and is somewhat jerky rather than constant. Calculations indicate that most rotary cone TCI bits have a common scraping pattern. The outermost row (heel row) on each cone usually scrapes toward the leading side and the next row inboard from the outermost row usually scrapes toward the trailing side. The two outer rows of the cutters combined account for more than 50% of the borehole area cut by most TCI rock bits. Therefore the design and function of this area of TCI bits is very critical.
Most of the TCI bits used for drilling soft to medium hard formations utilize tungsten carbide inserts having a chisel shape with an elongated crest at the top. Chisel shaped inserts are well known in the art of TCI bits. TCI bits utilizing inserts having elongated crests have generally been built with the lengthwise centerline of the crests relatively in line with the axis of the cone cutter. U.S. Pat. No. 4,393,948 to Fernandez teaches a relatively random orientation of the crests of inserts on cone cutters. Milled tooth bits have been built with the gage of one cone oblique to the leading side and the gage row of another cone oblique to the trailing side. This arrangement on milled tooth bits provides "cross hatched" impressions on the borehole bottom to minimize tracking. Tracking is detrimental drilling condition that develops when teeth from one cone fall into the impression of teeth made by another cone.